| MEK2 (MAP2K2) is a dual-specificity protein kinase of the MAP kinase kinase family that, together with its paralog MEK1, forms the central activating tier of the canonical RAS–RAF–MEK–ERK signaling cascade, relaying mitogen and growth factor signals to ERK1/2 to control proliferation, differentiation, survival and metabolism. The enzyme contains an N‑terminal regulatory region with docking sites for ERK and scaffolds, a bilobed protein kinase domain with the conserved activation segment, and a short C‑terminal tail; MEK1 and MEK2 share high sequence identity in their catalytic domains and adopt similar three-dimensional folds, as revealed by crystal structures of ternary complexes with MgATP and noncompetitive inhibitors that bind a unique pocket adjacent to the nucleotide site and lock the unphosphorylated enzymes into a closed yet catalytically inactive conformation. Activation of MEK2 occurs downstream of RAF kinases, which phosphorylate serine residues in its activation loop (analogous to MEK1 S218/S222) in a process coordinated by the KSR scaffold; once phosphorylated, MEK2 phosphorylates ERK1 (MAPK3) and ERK2 (MAPK1) on threonine and tyrosine in their TEY motifs, thereby triggering ERK nuclear translocation and transcriptional regulation of genes controlling cell-cycle progression, apoptosis and differentiation. Structural and biochemical studies indicate that MEK1 and MEK2 can form heterodimers, and a Raf-induced allosteric transition of KSR stimulates MEK phosphorylation, highlighting a multi-protein regulatory module in which MEK1/2 conformation, scaffold association and activation loop phosphorylation together determine ERK signal strength and duration. In human disease, aberrant activation of the RAF–MEK–ERK cascade, driven mainly by oncogenic RAF or RAS mutations, is detected in roughly one-third of cancers, and constitutive activation of MEK1 has been shown to transform cells, while expression of constitutively active MEK2 mutants similarly induces oncogenic phenotypes, validating MEK1/2 as critical “gatekeepers” of ERK activity in tumorigenesis. Although MEK2 mutations are relatively infrequent, germline MAP2K2 variants contribute to RASopathy syndromes such as Noonan syndrome, and allelic series in mice where Mek1 is selectively deleted on a Mek2-null background reveal isoform-specific roles: total loss of MEK1 and MEK2 is compatible with normal lifespan, but partial MEK dosage leads to glomerulonephritis or lymphoproliferative disorders due to dysregulated B‑ and T‑cell activation, demonstrating that fine-tuning of MEK2-containing ERK signaling is pivotal for limiting lymphocyte activation and autoimmunity. |
